Thermoelectric properties and scattering mechanisms in natural PbS

Abstract

X-ray diffraction and energy dispersive x-ray spectroscopic analyses showed a natural galena (PbS) crystal from Freiberg in Saxony (Germany) to be a single phase specimen [rock salt (NaCl) structure type, space group $Fm\overline{3}m, a=5.932(1)$ \AA{}] with stoichiometric composition and an enhanced dislocation density ($\ensuremath{\delta}\ensuremath{\approx}{10}^{11}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$). The latter parameter leads to an increase of the electrical resistivity in the high-temperature regime, as well as to the appearance of phonon resonance with a characteristic frequency ${\ensuremath{\omega}}_{\mathrm{PR}}=3.8(1)$ THz. Being in the same range (i.e., 3--5.5 THz) with the sulfur optical modes of highest group velocities, it results in a drastic reduction (by $\ensuremath{\sim}$ 75%) of thermal conductivity ($\ensuremath{\kappa}$) at lower temperatures (i.e., $<100$ K), as well as in the appearance of a characteristic minimum in $\ensuremath{\kappa}$ at $T\ensuremath{\approx}$ 30 K. Furthermore, the studied galena is characterized by phonon-drag behavior and by temperature dependent switch of the charge carrier scattering mechanism regime (i.e., scattering on dislocations for $T<100$ K, on acoustic phonons for 100 K $<T<170$ K and on both acoustic and optical phonons for 170 K $<T<300$ K). The combined theoretical calculation and optical spectroscopic study confirm this mineral to be a direct gap degenerate semiconductor. The possible origins of the second-order Raman spectrum are discussed.